Page 635 - Carrahers_Polymer_Chemistry,_Eighth_Edition

598 Carraher’s Polymer Chemistry

the female cavity side forcing the material against the male mold. Luggage and automotive parts are
made using this technique.
In pressure-bubble vacuum snapback the heated sheet is clamped and sealed against a pressure
box. Air is forced through the female pressure box forcing the sheet to push outward from the pres-
sure box. A male mold is then pressed against the bubble and as it pushes into the pressure box,
excess air is forced from the pressure box forcing the heated sheet to take the shape of the male
mold. The major difference between the vacuum snapback and pressure-bubble vacuum snapback
is that in the vacuum snapback process vacuum from a female pressure box distorts the sheet away
from the male mold and into the female pressure box, while in the pressure-bubble vacuum snap-
back excess pressure from the female pressure box forces the heated sheet toward the male mold and
away from the female pressure box.
Similar to pressure-bubble vacuum snapback forming, air slip forming seals a heated sheet to the
surface of a pressure chamber employing a male mold. It differs in the way the bubble is produced.
Here, the heated sheet is clamped above a male mold. Pressure against the mold is created by the
upward motion of the male mold toward the sheet causing it to bubble away from the oncoming
mold. At the right time, a vacuum is applied through the male mold that causes the sheet material
bubble to collapse and form about the male mold.
Trapped-sheet contact heat pressure forming utilizes a heating plate that contains many small
vacuum and air pressure holes. A sheet is placed between the heating plate and the female mold.
Initially, excess air pressure from the mold pushes the sheet into contact with the heating plate. The
heating plate then heats the sheet and after desired heating, vacuum is applied from the female and/
or pressure applied through the heating plate pulls/pushes the heated sheet material into the female
mold. Additional pressure can be used for trimming the article. Candy and cookie box liners and
some medical packaging is made using this process.
These bubble-associated processes are aimed at prestretching the heated sheet to allow more
even walls and bottoms to be formed.
In matched-mold forming a heated sheet is placed between a matched female and a male mold
parts. As the two mold halves close, they distort the sheet to their shape. The air between the
mold halves is removed. The article walls are more uniform than for many of the thermoforming
techniques. This technique is used for the production of foamed PS and foamed polyolefi ns food
containers.
A number of other techniques have been developed either to handle special materials or to create
specifi c articles.

18.10 CASTING

Casting is employed in making special shapes, sheets, films, tubes, and rods from both thermoplas-

tic and thermoset materials. The essential difference between most molding processes and casting
is that no added pressure is employed in casting. In casting, the polymer or prepolymer is heated to

a fluid, poured into a mold, cured at a specific temperature, and removed for the mold. Casting of

films and sheets can be done on a wheel or belt or by precipitation. In the case of a wheel or belt, the
polymer is spread to the desired thickness onto a moving belt as the temperature is increased. The
film is dried and then stripped off. “Drying” may occur though solvent evaporation, polymerization,

or cross-linking.
18.11 EXTRUSION
Extrusion involves a number of processing operations and is widely used. We will look at extru-
sion as it is involved in several of these processes. These processing operations are used together
or separately. A representative extruder is shown in Figure 18.15. The extruder accepts granulated
thermoplastic in a hopper (c), and forces it from the feed throat (d) through a die (f). The die may

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